# Do we need dark matter and dark energy, if the behaviour of the universe in its initial stages was similar to that of the Sun? [closed]

According to CMBR the universe was a cloud of plasma and was a perfect black body, $$380,\!000$$ years after big bang.

But the Sun in our solar system also is in the state of plasma, thus making it a blackbody. So it is possible that the universe in its initial stage also behaved similarly(I.e. radiated the energy produced as a result of fusion reactions during the recombination epoch beyond the boundary of the plasma). And this is the reason we find find less baryonic matter than we should. Thus making the concept of dark matter irrelevant.

• Black bodies DO radiate; dark matter is not black, because it does not radiate. Thermodynamic principles require surfaces which absorb (are black) be good radiators of light, which is a feature absent in dark matter. – Whit3rd Apr 15 '19 at 2:15
• Dark matter and dark energy are not the same thing – lcv Apr 15 '19 at 3:54
• I don't see how your conclusions follow from your premises. And that's ignoring that your premises aren't correct. And given how you mix up dark matter and dark energy, not to mention the baryon asymmetry (which has nothing to do with either) I'm quite interested in the way you arrived at your conclusion. How did you come to think that either of these three are in any way related? – Luaan Apr 15 '19 at 7:08
• You seem to be mixing up dark matter, dark energy, the baryon asymmetry, and blackbodies (which are almost the exact opposite of dark matter) into one big soup -- it's hard to tell what your logic is. – knzhou Apr 15 '19 at 12:46
• @eromod Uh... no offense, that seems to be a near total reversal of what the video actually says. I know that skepticism of dark matter is very popular in popsci these days, but still. – knzhou Apr 15 '19 at 12:54

You should look in to the history behind the dark matter hypothesis. It started not from the examination of cosmology and the CMB, but from the motion of galaxies in clusters and stars orbiting around galaxies. See, the vast majority of ordinary matter in every galaxy is contained in the gas between the stars, not the stars themselves. Because of that, we can get a decent handle on how much ordinary matter is around by observing that. Tools for this purpose: the 21 cm line of atomic hydrogen, when the gas is cold, as much of it is in spiral galaxies, and looking at the x-ray spectrum when it is exceptionally hot, as it is between galaxies in large clusters.

When we examine the way the parts of galaxies, and the galaxies in clusters, move, they're travelling way too fast. If the mass we can see direct evidence for is all there is, the clusters would not be able to hold on to their hot gas and galaxies, and the galaxies would not hold together, either.

"So what? Maybe the matter is there, it just isn't giving off light." Trouble is, if it were hot enough to be a plasma, and thus lack spectral lines, we could see it directly. If it were too cold to be a plasma, it would block light from galaxies and quasars in the background more in the matter's spectral lines. So whatever is producing this extra gravity has to neither emit nor absorb light in any way we've been able to detect.

It just so happens that adding dark matter (or something very like it) to the cosmology simulations is also essential to explain the CMB data.

Now, you may object that the extra gravity we've observed may have some other source. For instance, maybe Newton's law of gravitation is simply wrong on the scale of galaxies and larger. The trouble that idea runs into is you're no longer able to explain the bullet cluster, where the gas between the galaxies has collided, but the dark matter and galaxies did not.

On the subject of black body spectra. The black body is the spectrum that a gas of photons assumes if it is in thermal equilibrium (constant uniform temperature everywhere). The CMB is very very nearly a black-body because at the time the differences in temperature between any two parts of the universe were very very small. The sun, however, is surrounded by a very cold vacuum, and that lack of equilibrium will inevitably cause the spectrum to deviate from the ideal Planck function.

• @0x90 Not likely. The way we measure mass is based on the physics of how light interacts with matter, something we have measured extremely well here on the ground. See, if a gas is thin enough that it is unlikely to reabsorb anything it emits ("optically thin" - see the 21 cm line), then we can deduce the gas temperature from the way the shape of the line is distorted, and the amount of gas along the line of sight ("column density") from brightness. Total mass is just adding up the lines of sight. The process is similar for absorption, just inverted. – Sean E. Lake Apr 15 '19 at 5:45
• @0x90 That still leaves the question of what could that matter possibly be. Normal atoms and molecules aren't perfectly transparent - plasma even less so. We see the galaxy illuminated in many wavelengths of light. If there is extra mass out there (it's entirely possible there isn't - keep in mind that the calculations are entirely classical, and it's possible e.g. a quantum theory of inertia and/or gravity will correct better than dark matter), it really seems like it doesn't interact with light. That's not really weird on its own - particles don't have to interact electromagnetically. – Luaan Apr 15 '19 at 7:13
• @JollyJoker Dark matter is a hypothesis that lacks some specifics. Dark matter, for example, is not modified gravity. In other words, when we say "dark matter" we really do mean that it is some kind of "matter" - it obeys F=ma, is constructed of particles that have mass, etc. In the language of particle physics, we'd say that dark matter is composed of excitations in some field (probably fermionic). Modifed gravity, on the other hand, is a change to how the gravitational field behaves, no other fields involved. Crucially, dark matter and regular matter are separable. – Sean E. Lake Apr 15 '19 at 7:45
• @SeanE.Lake: Just wanted to say thanks for your patient and thorough answer and replies. I have literally never seen anyone explain dark matter so helpfully to laymen. – user541686 Apr 15 '19 at 8:27
• @SeanE.Lake I think that goes a little too far -- at least, it's not how people in the field use the word. For example, "axion dark matter" is quite a popular field of research, but axions are bosonic. – knzhou Apr 15 '19 at 15:19

But the Sun in our solar system also is in the state of plasma and yet doesn't act like a blackbody

Wrong, the sun radiation is approximately fitted as a black body. The word "black body" does not describe the frequencies, but the assumption that it absorbs all radiation falling on it and re-emits it.

Here is the sun, and it fits the black body formula approximately.

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

Plasma is also described by black body radiation.

so it is possible that the universe in its initial stage also behaved similarly. And this is the reason we find find less baryonic matter than we should. Thus making the concept of dark energy irrelevant.

This is wrong, as seen above because your premiss is wrong, but also, dark matter is necessary to fit the newtonian rotational curves of galaxies, and more observational evidence can be found in this link..

Dark matter is a completely classical observation of newtonian and general relativity. Baryon asymmetry comes from quantum mechanical knowledge of the content of the classical masses, and does not involve dark matter in any meaningful manner.

• The resolution of your picture is too small, I can't read it. – Azzinoth Apr 15 '19 at 13:22
• @Azzinoth sorry, I thought I had given the link. If you click on the image in the link a large version appears – anna v Apr 15 '19 at 15:01